iBiquity Digital Corporation's HD Radio™ system is designed to permit a smooth evolution from current analog amplitude modulation (AM) and frequency modulation (FM) radio to a fully digital in-band on-channel (IBOC) system. This system delivers digital audio and data services to mobile, portable, and fixed receivers from terrestrial transmitters in the existing medium frequency (MF) and very high frequency (VHF) radio bands. Broadcasters may continue to transmit analog AM and FM simultaneously with the new, higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog to digital radio while maintaining their current frequency allocations. Examples of waveforms for an FM HD Radio system are shown in U.S. Pat. No. 7,724,850, which is hereby incorporated by reference.
A variety of antenna diversity techniques have been developed and deployed for use with automotive FM receivers. They are used to mitigate the effects of distortion and outages due to multipath propagation of the received FM signal, and can also accommodate the directional characteristics of glass-embedded window antennas. All diversity techniques use two or more antenna elements, and some require multiple tuners/receivers. Some techniques can be applied to digital signals, and some cannot.
Blind diversity switching can be economically attractive because a simple multi-position switch connects the selected antenna element to only one tuner and receiver. However, because the switching is blind, there is no guarantee that the next antenna element will carry a better signal, and subsequent switching may occur in rapid succession until a good signal is found. Furthermore, since the digital signal is coherently detected and tracked, each antenna switching event is likely to cause symbol corruption and temporary loss in channel state information (CSI) and coherent tracking.
Such switching transients can be avoided by using a smooth diversity combining algorithm. These techniques involve some kind of multiple-input signal combining (pre or post-detection), and require multiple tuners. One combining method for analog FM signals employs phase diversity using a constant-modulus algorithm (CMA). However, this approach is not valid for HD Radio signals as the digital sidebands are not characterized by a constant envelope.
IBOC HD Radio receivers can be used in combination with switch diversity antenna systems. However the use of switch diversity antennas introduces abrupt transients in the coherent tracking of the digital signal, which degrades digital performance.